The long-term goal of this proposal is to obtain a better understanding of the functional role of steroidogenesis in the developing rodent yolk sac placenta. To accomplish this goal, we propose to investigate the nature of the pathways for steroid metabolism, the developmental pattern of steroid-metabolizing enzymes and the factors which regulate steroidogenesis in both the parietal and visceral portions of the yolk sac placenta. Pregnant rats at various stages of gestation will be used in these studies. Homogenates of the parietal and visceral yolk sacs will be assayed for a variety of steroid-metabolizing enzymes, including: 3-beta-hydroxysteroid dehydrogenase, delta 5 yields delta 4 isomerase, 5-alpha-reductase, 17-beta-hydroxysteroid dehydrogenase, 17-alpha-hydroxylase, 17-20 lyase, 3-alpha-hydroxysteroid dehydrogenase, 20-alpha-hydroxysteroid dehydrogenase and aromatase. The assays will be carried out by incubating tritium-labeled steroid substrates in the presence of appropriate cofactors (NAD ion, NADPH, etc.) and then isolating newly synthesized radioactive steroid products by thin layer chromatography. Using the methods described, the uptake, biosynthesis and metabolism of steroid hormones also will be studied in early post-implantation whole-rat embryo explants grown in culture during the period of organogenesis.